Quantitative thermodynamic understanding of the solid-liquid interface during heterogenous catalysis
Open Access
- Author:
- Binz, Jason Michael
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 16, 2013
- Committee Members:
- Robert Martin Rioux Jr., Dissertation Advisor/Co-Advisor
Darrell Velegol, Committee Member
Themis Matsoukas, Committee Member
James Hansell Adair, Special Member - Keywords:
- heterogeneous catalysis
catalyst synthesis
strong electrostatic adsorption
isothermal titration calorimetry - Abstract:
- We use isothermal titration calorimetry (ITC) to study the binding mechanism that occurs at the solid-liquid interface during heterogeneous catalyst synthesis. The study focuses on Pt transition metal complexes (TMC) that adsorb via an electrostatic interaction onto an oxide support (γ-Al2O3 and SiO2). The electrostatic driving force is controlled by adjusting the solution pH during synthesis further from its point-of-zero charge (PZC). γ-Al2O3 was chosen due the amphoteric nature of the surface potential (PZC = 7.8), which will allow for a similar pH range to be probed for anion (chloroplatinic acid (CPA)) and cation (tetraammineplatinum(II) nitrate (TAP)) adsorption. SiO2 was chosen because it is an acidic support (PZC = 4.4) which will allow a large pH range for TAP adsorption. For all synthesis conditions, we analyze the thermodynamic parameters (K, ΔG, ΔS, and ΔH) and the binding stoichiometry to determine the effect of changes in the electrostatic driving force on final catalyst properties. We correlate the thermodynamics with Pt nanoparticle size and loading through the use of high-energy x-ray diffraction with a PDF analysis over a temperature range of 323 – 773 K, determine the final nanoparticle size after reduction at 623 K with the use of chemisorption, and elemental analysis to determine weight loading. For CPA and TAP adsorption close the PZC, we observe minimal TMC uptake that exhibit small exothermic enthalpies, which correlate to larger supported nanoparticles. As the solution pH is shifted further from the PZC, the supported nanoparticles are smaller with a higher weight loading and an increased exothermic enthalpy. There is also a large difference noticed between the adsorption enthalpy for CPA and TAP. We suggest this could be due to the formation of covalent bonding between the CPA and surface hydroxyl groups once CPA is in the diffuse layer or could be associated with larger hydration sheath interactions between TAP and the surface. We also observe the free energy of adsorption remains relatively constant through these studies, suggesting there are compensating entropic contributions associated with interactions between the hydration sheaths and surface water groups. We also use ITC to study the adsorption of Rh(OH)3 nanoparticles onto early transition metal oxides. This study highlights the ability of the ITC to elucidate specific binding enthalpies from a complex reaction network through the use of the Born-Haber cycle.